IRIX 5.3 and resetting root password

OK, this is driving me nuts. I picked up an SGI Iris Indigo the other day which seems to run quite happily (as seems compulsory with old SGI machines, it did *not* come with a keyboard). I don't however know the root password. I do have another system SCSI disk for which I know the root password, so I can boot with that and the 'new' drive as a secondary disk, then mount and access the new disk's root partition from there. Right, the 'new' machine has IRIX 5.3 installed, and is using shadow passwords. So, first I tried editing the passwd file and just clearing the password field for root's entry (which was just set to 'x', presumably to signify the use of a shadow password), then booting from the 'new' disk with the cleared password. That didn't work - it just gave me an invalid login message. So instead I tried the same, but cleared the encrypted password string in the /etc/shadow file. Still no luck. I've now tried with and without the 'x' present in the passwd file, and with and without the encrypted password string in the shadow file. A few possibilities spring to mind: 1) IRIX 5.3 doesn't allow direct root login on the X console? 2) The system's set up to use some sort of authentication other than /etc/passwd - any pointers for what to look for if so? 3) The system's set to read from a file other then /etc/shadow - no idea where this is set up if so. I did quite a bit of work using newer SGI systems with IRIX 6.5.3 a few years ago, but I'm pretty rusty there - plus I've never seriously used earlier IRIX releases. Oh, anyone have install media for IRIX 4.0.5 and 5.3? (Our first Indigo's running 4.0.5). Be nice to be able to rebuild the machines if/when the OS drives fail! cheers, Jules